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Wednesday, March 12, 2014

Tiny, Portable Turntable Strobe

I love turntables. The very idea is simultaneously insane and an awesome feat of engineering.

Microscopic grooves in a rotating disc of plastic vibrate a minuscule, precisely ground chunk of diamond stuck on a tiny metal tube with magnets attached, inducing tiny currents in a cartridge coil, subsequently amplified up the wazoo. Somehow, instead of sounding like garbled crap, the reproduced sound is quite excellent on a decent system.

But how fast does the record spin? That's the purpose of my project, an extremely accurate, compact turntable strobe. An LP is supposed to spin at 33-1/3 RPM. Many probably don't. Not precisely. And the voices in my head like precision.

Introducing Pocket Turntable Strobe

Some turntables, like my Realistic Lab 400 above, have strobes built in that run off mains frequency to illuminate precisely spaced platter markings that appear to stand still when the platter speed is just right. Except the mains frequency varies and is only 60Hz on average.

Compare to my Turntable Strobe, which uses a quartz crystal and ATtiny25 to flash a white LED at 60.0±0.03Hz (30ppm absolute accuracy plus 20ppm temperature variation). Use it with platter strobe markings or speed check disks that you can find online.

The circuit is simple, requiring two capacitors and two diodes and one of the pins on the microcontroller for the clock signal. The result is a higher voltage supply (4.2V unloaded) that can brightly illuminate the high intensity, 5mm LED through a 1K current limiting resistor. I'll provide more detail on this circuit in an upcoming article.

Another trick to minimize complexity and parts count is that the microcontroller directly sinks current from the LED. You may object, claiming it's unsafe given the LED circuit is powered by a voltage exceeding the Vcc+0.5V limits of the ATtiny. Rules are meant to be broken, provided you have sufficient understanding of their purpose and how things work. I'll show you why it's safe in another upcoming article.